Built up rotor assemblies for vacuum pumps

ABSTRACT

A composite impeller assembly for use in a high vacuum pump operating in the free-molecule flow pressure range. The impeller assembly comprises left and right hand impeller units each made from a plurality of rotor assemblies held in closely abutting relationship by the cooperating relation between at least one fastener and a central core shaft extending through each of the rotors. Each rotor includes a row of blades and a hub portion with front and rear annular contact surfaces thereon, and a center portion extending inwardly and terminating adjacent a central opening for receiving the shaft. The effect of the builtup construction is that the contact surfaces provide a large effective impeller hub diameter for maximum stiffness against deflection, and the relatively small diameter center openings reduce stresses which would be present with an opening of an order equal to the diameter of the hubs just inside the contact surfaces.

United States Patent Schulte, deceased [54] BUILT UP ROTOR ASSEMBLIES FOR VACUUM PUMPS [72] Inventor: Charles A. Schulte, deceased, late of Lexington, Mass. 02173 by Grace Schulte, executrix [22] Filed: Oct. 27, 1969 [21] Appl. No.: 869,968

[52] U.S.Cl ..416/l99,416/244 [51] Int. Cl ..FOld 5/06 [58] Field ofSearch ..416/198, 199, 201, 229, 244; 415/90, 93, 198,199

[56] References Cited UNITED STATES PATENTS 2,461,242 2/1949 Soderberg ..416/201 3,070,348 12/1962 Vogel ..416/198 A 843,237 2/1907 Shephercl.. ..415/93 1,551,402 8/1925 Junggren ..416/198 2,317,092 4/1943 Allen ..416/198 X 2,356,605 8/1944 Meininghaus ..416/198 2,440,933 5/1948 Cunningham ..416/201 X 2,637,521 5/1953 Constantine et al. ..416/201 X 3,399,827 9/1968 Schwartzman ..415/90 X 3,659,958 [4 1 May 2,1972

FOREIGN PATENTS OR APPLICATIONS Germany ..416/201 France ..416/198 [57] ABSTRACT A composite impeller assembly for use in a high vacuum pump operating in the free-molecule flow pressure range. The impeller assembly comprises left and right hand impeller units each made from a plurality of rotor assemblies held in closely abutting relationship by the cooperating relation between at least one fastener and a central core shaft extending through each of the rotors. Each rotor includes a row of blades and a hub portion with front and rear annular contact surfaces thereon, and a center portion extending inwardly and terminating adjacent a central opening for receiving the shaft. The effect of the built-up construction is that the contact surfaces provide a large effective impeller hub diameter for maximum stiffness against deflection, and the relatively small diameter center openings reduce stresses which would be present with an opening of an order equal to the diameter of the hubs just inside the contact surfaces.

6 Claims, 4 Drawing Figures Patented May 2, 1972 3,659,958

2 Sheets-Sheet l INVEN TOR CHARLES SCHULTE DEC'D.

Wiifj BY R:

ATT'Y.

Patented May 2, 1972 2 Sheets-Sheet 2 MC MIDEQ. TC Eu ,Y M H W f A N U m S.

M? M m BUILT UP ROTOR ASSEMBLIES FOR VACUUM PUMPS BACKGROUND AND DESCRIPTION OF THE INVENTION The present invention relates to so-called turbomolecular pumps, that is, high vacuum pumps operable in pressure ranges wherein the mean free path of gas molecules is relatively large compared to any significant dimension of blades contained on the stators or rotors of an axial flow, turbine type pump. Typically, a pump of the type in question comprises a housing having right and left hand turbine units therein, each composed of a number of stages, with each stage consisting of a rotor and an associated stator. The right and left hand units are identical except that they are arranged for opposite directional movement of molecules impinging thereon with the same direction of rotation of the impeller. The inlet is disposed between the left and right hand turbines and an outlet is disposed adjacent each axial end thereof.

In keeping with known practices, impeller assemblies such as these, in order to provide the necessary blade tip speed, are rotated at extremely high rates, typically 10,000 to 20,000 revolutions per minute (r.p.m.) or more. At such speeds, and particularly considering the substantial axial extent of such impellers in relation to the diameter thereof, dynamic balance is very critical. Thus, an impeller assembly for turbomolecular pump must be capable of withstanding the stresses imposed on it by high speed rotation, including not only stress of centrifugal force, but the stresses imposed by dynamic imbalance. Dynamic imbalance, in structures of insufficient rigidity, is often not properly damped out, but is often amplified by harmonic vibrations to an extent that significant distortion in the impeller occurs, creating risk of undesired vibration and even impeller breakage. Accordingly, the need for an impeller shaft which is sufficiently stiff to obviate these conditions is indicated. Stiffness in such impellers is normally obtained by providing a hollow shaft of the largest possible diameter consistent with the size of the blades which must be attached to the periphery thereof, with the wall thickness ofthe shaft being relatively thin for reduced weight, to the largest practical extent. The hollow center is also desirable since significant bending stiffness is not afforded by the innermost portions of a solid shaft.

However, in the case of a cylindrical shaft, or section thereof, great stress concentration is imposed by centrifugal force at the inside diameter thereof, it being known that greater stress concentrations occur at a larger diameter opening than at a smaller diameter opening, under the same conditions of load.

As a result of these considerations, impeller assemblies for use in high vacuum pumps of the prior art have been of designs having the disadvantages of insufficient stiffness, excess weight, undue stress concentrations, or the like. On the other hand, where such impellers have been characterized by relatively good high speed performance characteristics, they have been constructed of exotic and expensive materials, or have been made by complex expensive methods, or both.

Accordingly, an object of the invention is to provide an improved impeller assembly for use in high vacuum pumps.

Another object is to provide an impeller construction wherein a plurality of individual rotor elements are held together by a single axial shaft in a manner so that maximum rigidity and stiffness are provided.

Another object is to provide a built-up construction for a turbine pump impeller in which stress concentrations on rotating parts are held to a minimum.

Another object is to provide an impeller construction in which a pair of built-up impeller units are disposed on a shaft and spaced apart by a cylinder interposed therebetween.

Another object is to provide an impeller construction in which each rotor includes a small central opening, a pair of increased diameter annular contact faces and a row of blades disposed outwardly radially thereof, with all the contact faces being held in tightly abutting relation by fastening means attached to a central core shaft passing axially through such openings.

Another object is to provide a built-up impeller construction which may be used for rotors having integrally formed or separately formed blades comprising a part thereof.

The above and other objects and advantages are accomplished by providing an impeller assembly comprising a central core shaft, a plurality of rotor elements disposed thereover with annular contact face openings on each rotor element contacting abutting face portions of adjacent rotors, with each rotor including a reduced diameter opening centrally thereof through which the core shaft extends, and with fastening means on the core shaft tightly locking the elements in place.

The exact manner in which these objects and other inherent objects and advantages of the invention are accomplished will become more clearly apparent when considered in connection with the following detailed description of the preferred embodirnents of the invention and when reference is made to the accompanying drawings, in which like reference numerals indicate corresponding parts throughout.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view, partly in elevation and partly in section, showing a vacuum pump and a portion of an impeller therefor made according to the present invention;

FIG. 2 is a fragmentary enlarged view of the built-up impeller assembly of the invention, partly in section and partly in elevation, and showing the right hand turbine portion of a double ended impeller; and

FIG. 3 is an enlarged, fragmentary exploded view, partly in elevation and partly in section, showing the built-up impeller of the invention including some of the components and the manner of assembly thereof;

FIG. 4 is a fragmentary front view of a portion of a built-up rotor assembly of the invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS OF THE INVENTION Although the invention is applicable to other particular constructions, reference herein will be made to an embodiment of the invention in which a double ended impeller unit is provided, that is, a unit in which the pump includes right and left hand turbine portions, each composed of a stator assembly and an impeller assembly having a plurality of rotor elements thereon. Gas flow is from a central portion of the pump in opposite axial directions, entering the pump housing in the middle and being exhausted on either end of the pump unit. Since it is understood that the right and left hand portions are identical except that they are of mirror image configuration in relation to each other so that gases will pass out opposite ends when the impeller rotates in one direction, only one end of the impeller will be described in detail, it being understood that the other end thereof is substantially identical thereto but of opposite hand rotation.

Referring now particularly to FIG. 1, a vacuum pump 20 is shown having a flange 22 for association with an area to be evacuated, a scroll or torus cover 24, and a cylindrical pump casing 26 having end portions 28 adapted to support a bearing 30 on the end of an impeller shaft for rotation thereof inside the casing 26. A driven gear 34 is adapted to be engaged with a driving gear (not shown) to cause rotation of the shaft 32 in a known manner. A number of spacers 36 are disposed inside the housing 26 and the spacers 36 serve to locate and support a plurality of stator elements 38, each having a plurality of blades disposed thereon in a known manner. A turbine assembly 40 is formed at the left hand portion of the pump 20 by the rotor units 42 and the stators 38 interleaved therebetween. In the illustration, the lowest pressure or highest vacuum portion of the pump 20 is in the region designated 44, lying adjacent the axially inwardly disposed stages of the turbine 40, whereas the higher pressure stages 46 are disposed adjacent the outlet annulus 48. Typically, the impeller assembly 50 of such a pump might rotate at speeds of 4,000 up to 20,000 r.p.m., and might produce a vacuum as low as to 10 torr. or more. Obviously such speeds require significant strength and balance in the impeller.

Referring now to FIG. 2, the impeller assembly 50 is shown to include a core shaft 52, having a splined outer end portion 54 thereof lying outwardly of a nut 56 holding a locking member 58 in place adjacent an axially outermost rotor element 60. Disposed in the axial center of the impeller 50 is a cylindrical spacer 62 having an annular contact surface 54 on an axial face thereof, a radially inwardly extending flange 66 afiixed to an inner surface 68 thereof, and an outer, blade free surface 70, effectively serving as the inner surface of the highest vacuum portion 44 of the pump 20.

Referring now also to FIG. 3, the rotatable, built-up turbine assembly 40 is shown to include a plurality of rotor units 42, and it may be seen that these units include a row of blades 72, and a hub portion 74, with the hub portion 74 having a peripheral part 76 thereof joined to the blades 72, an intermediate hub body portion 78 having oppositely disposed axial contact surfaces 80, 82 thereon, and a hub center portion 84 lying inwardly thereof and including inner marginal surfaces 86 defining an opening 88 in the center thereof. The center portion 84 is partially defined by oppositely disposed axial surfaces 90, 92 which are of reduced axial extent with respect to the hub body portion 78 lying between contact surfaces 80, 82. Thus, in an assembled position of use, each contact surface 80 is in tightly abutting contact with an adjacent oppositely disposed surface 82, while the surfaces 90, 92 disposed inwardly thereof are free from contact with their oppositely disposed counterparts. Only a slight working clearance is provided between the outer diameter of the shaft 52 and the opening 88 defined by the inner margins 86 of the center portion 84. Referring again to FIG. 3, it will be seen that the locking member 58 includes an enlarged diameter, cylindrical portion 94 which includes a contact surface 96 adapted to abut the contact surface 82 on the adjacent rotor 42 and be coextensive with the surface 82.

Referring now to FIGS. 3 and 4, it is shown that each blade 72 in the axially innermost core first stages is a segmented blade having outer, intermediate and inward segments 98, 100, 102 respectively. Each segment typically comprises a flat face portion 104 and an angularly inclined working face portion 106. By reference to FIG. 3, it will be noted that the blades 72 are integrally formed with the peripheral portion 76 of the hub 74, and that accordingly, the rotors 42 are of a one piece construction. Some of the rotors 42, such as those shown at the right in FIG. 3, may have working faces formed only in the tip portions 108 thereof, with the result that the blades 72 taper somewhat inwardly toward the radially outermost portions thereof. However, blades of this type, as well as those having any greater or lesser number of segments may also be integrally formed, if desired.

FIG. 4 illustrates a construction in which the blades 72 are formed separately from the hub 74, and in such case, the blades 72 are joined to the peripheral portion 76 of the hub 74 by the provision of splined roots 110 having axial faces 112 which are parallel to the contact surfaces 80, 82 on the intermediate body portion 78 of the hub 74. Thus, in the illustrated construction, the splined roots 110 serve to secure the blades against radial movement, and additional locking force may be exerted thereon in an axial direction by contact with an adjacent rotor 42. The root portion 110 need not lie within the intermediate portion 78, but it may be desirable to adopt this construction, particularly if additional axial locking force is desired.

The root portion 1 10 has been shown as being splined, but it will be understood that interlocking of parts may be accomplished by provision of a simple T-shaped root section or the like.

Referring now to FIG. 2, it can be seen that with the fastener 56 in place, the construction of the impeller 50 is such that, in efl'ept, a cylindrical shaft is provided havin an effective outside diameter equal to the diameter of the sli oulder portions 114 forming the outermost portion of the intermediate hub body portion 78. The reduced axial extent of the portion of the rotor lying between faces 90, 92 in relation to the extent of the portion between faces 80, 82 insures that there will be no contact radially inwardly of the surfaces 80. 82 and therefore, the contact pressures are confined to the areas of the contact surfaces 80, 82. The provision of surface 54 on the spacer 62 also creates this eflect. As pointed out earlier, it is also an advantage of this construction that the opening 88 is small with relation to the effective diameter of the built-up shaft, and therefore, surfaces 90, 92 are necessary even though they are not in contact with their counterparts.

Shafts made according to the present invention have displayed the advantages of stifiness associated with large diameter shaft constructions, ease of balance and assembly made possible by individual rotor construction, and lower cost brought about by use of the small core shaft and non-exotic materials and forming techniques.

Built-up impeller assemblies made according to the present invention are capable of sustained operation in the range of l6,000-20,000 r.p.m. or more without exhibiting undue vibration, flexibility or the like, even when constructed of materials which are easy to fabricate and assemble. It will thus be seen that the present invention provides a composite impeller assembly for vacuum pumps having a number of advantages and characteristics, including those hereinbefore pointed out, and others which are inherent in the invention.

1 claim:

1. A composite impeller assembly for use in a high vacuum axially split flow turbomolecular pump having a generally centrally disposed inlet, said impeller assembly comprising, in combination:

a central core shaft of a given diameter;

a plurality of rotor elements disposed in axially aligned relation over said shaft;

a cylindrical spacer disposed between said first and second impeller sections having opposed axially directed contact surfaces adapted to respectively engage the contact surfaces on the rotor elements of said first and second impeller sections which are nearest said inlet; and,

a removable fastener on at least one end of said shaft, said fastener including an axially directed annular contact surface adapted to engage one contact surface on one of said rotor assemblies, said fastener urging all of said rotor assemblies into a tightly abutting relation with said contact surfaces abutting one another.

2. An impeller assembly as defined in claim 1 in which said blades are formed separately from said hub portions and in which a plurality of said blades are mechanically locked in place within each associated hub in position of use.

3. An impeller assembly as defined in claim 2 in which said blades include root portions received in said hub body portion and in which said annular contact surfaces are formed in part by said root portions of said blades.

4. An impeller assembly as defined in claim 1 wherein said opening closely surrounds said central core shaft, being spaced radially apart therefrom by only a slight working clearance.

5. The composite impeller assembly of claim 1 wherein the annular contact surfaces of the intermediate hub body portions of each of said rotor elements is of substantially constant diameter.

6. The composite impeller assembly of claim 1 wherein each of said rotor elements is of unitary construction.

UNITED STATES PATENT OFFICE CERTIFICATE OF CGRECTIQN Patent No. 3,659,958 Dated May 2, 197-2 v lnventofls) Charles A. Schu-lte, deceased It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 1, line 29, after "only" insert the- Column 1, line 32, after 'fextent" insert such- Column 2, line 52, after "opposite" insert -axial--' Column 4, line 40, after "shaft; insert the following paragraphs:

each of said rotor elements including a radially outermost blade row portion comprised of a plurality of circumferentially disposed impeller blades which have face portions angularly disposed with respect to the plane of rotation thereof, a rotor hub portion including a peripheral hub portion fixedly joined to said blades, an annular intermediate hub body portion having a given axial thickness and including axially forwardly and rearwardly facing annular contact surfaces of a substantial radial extent, and an annular hub center portion integrally formed with said intermediate hub body portions, said center hub portion extending substantially radially inwardly from said body portion and being of reduced axial extent relative to the thickness of said intermediate hub bodywherein contact between axially adjacent rotor elements radially inwardly of said annular contact surfaces is avoided, said center portion including a center opening defined by the inner margins of said center portion for receiving said central core shaft in closely spaced relationship therewith;

(continued on page 2) (10-59) uscoMM-oc eoan-Pesa a U.S, GOVERNMENT PRINTING OFFICE I969 365-334,

Patent No. 3,659, 958 Dated May 2, 1972 lnventor(s) Charles A. Schulte, deceased It is certifiedvthat error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

(continued) a-portion of said plurality of rotor elements forming a first impeller section and another portion of said rotor elements forming a second impeller section, said first and second impeller sections being axially spaced apart adjacent the inlet of said turbomolecular pump and having the blades thereof disposed for directing gas molecules contacted thereby in opposite axial directions away. from said inlet upon rotation of said impeller assembly in a given direction;

Signed and sealed this 8th day of May 1973.

(SEAL) Attest:

EDWARD M. FLETCHER,JR. ROBERT GOTTSCHALK Attesting Officer Commissioner of Patents Page 2 ORM PO- uscoMM-Dc some-ps9 U.S. GOVERNMENT PRINTING OFFICE: 1969 0-366-33L 

1. A composite impeller assembly for use in a high vacuum axially split flow turbomolecular pump having a generally centrally disposed inlet, said impeller assembly comprising, in cOmbination: a central core shaft of a given diameter; a plurality of rotor elements disposed in axially aligned relation over said shaft; a cylindrical spacer disposed between said first and second impeller sections having opposed axially directed contact surfaces adapted to respectively engage the contact surfaces on the rotor elements of said first and second impeller sections which are nearest said inlet; and, a removable fastener on at least one end of said shaft, said fastener including an axially directed annular contact surface adapted to engage one contact surface on one of said rotor assemblies, said fastener urging all of said rotor assemblies into a tightly abutting relation with said contact surfaces abutting one another.
 2. An impeller assembly as defined in claim 1 in which said blades are formed separately from said hub portions and in which a plurality of said blades are mechanically locked in place within each associated hub in position of use.
 3. An impeller assembly as defined in claim 2 in which said blades include root portions received in said hub body portion and in which said annular contact surfaces are formed in part by said root portions of said blades.
 4. An impeller assembly as defined in claim 1 wherein said opening closely surrounds said central core shaft, being spaced radially apart therefrom by only a slight working clearance.
 5. The composite impeller assembly of claim 1 wherein the annular contact surfaces of the intermediate hub body portions of each of said rotor elements is of substantially constant diameter.
 6. The composite impeller assembly of claim 1 wherein each of said rotor elements is of unitary construction. 